Process for making gold powder

ABSTRACT

A process for producing precipitated gold powder comprised of gold flakes, gold spheres or mixtures thereof by reducing a gold salt solution with selected reducing agents, in the presence of a protective colloid, at an elevated temperature. The process parameters provide a means for controlling the size and shape of the precipitated gold powder which has specific applications in the decorative and/or electronics industries (i.e., metallizing compositions).

United StatesPatent n 1 Short-ct al.

[ 1 Apr.3,1973

[54] PROCESS FOR MAKING GOLD POWDER [75] Inventors: Oliver A. Short;Richard V. Weaver,

both of Wilmington, Del.

[73] Assignee: E. l. du Pont de Nemours and Company, Wilmington, Del. 22Filed: Jul 2, 1971 [21] Appl.No.: 159,486

Related U.S. Application Data [63] Continuation-in-part of Ser. No.l5,2l2, Feb. 27,

1970, abandoned.

[52] U.S. Cl. ..7S/.5 A, 75/118 [51] Int. Cl. ..C22b 11/04 [58] Field ofSearch ..75/.5 A, US

[56] References Cited UNITED STATES PATENTS 3,201,223 8/1965 Cuhra etal. ..75/.5 A 3,345,l58 10/1967 Block et al ..75/.5 A p 3,539,114ll/l970 Short ..106/290 Primary Examiner-W. W. Stallard Attorney-John J.Kocko, [II

[57] ABSTRACT 19 Claims, 130 Drawings PROCESS FOR MAKING GOLD POWDERCROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-partof Ser. No. 15,212,

Gold powders are used in a variety of commercial products, includingdecorative compositions, electrical conductors and resistors, etc. Eachof these compositions require a gold powder having certaincharacteristics suitable for the particular use.

There are many chemical methods of producing gold powders and each onemay include variations involving pI-I, dilution, and temperature, amongothers. A frequently used technique is to precipitate gold fromchloroauric acid solution. This may be accomplished by use of activemetals such as zinc, magnesium, iron, cadmium, lead, and bismuth. Alsoeffective are inorganic reducing agents, such as ferrous sulfate, sodiumsulfite, potassium sulfite, sulfur dioxide and hydrogen peroxide, ororganic reducing agents, such as formic acid and formaldehyde.

The characteristics of the gold powder, such as surface area, particlesize, particle shape, and tendency to adsorb oil, among others, aredependent both on the conditions of precipitation and on the particularreducing agent used. Physical characteristics such as these influencethe chemical processability and determine to a large extent theappearance, usefulness, and efficiency of the gold powder in particularapplications. Thus it is highly desirable to be able to control thephysical characteristics of the powder by skillful manipulation of thevariables of the precipitation operation.

Poor appearance, manifested by a low level of brightness, and poorfireability of gold-printed metallizing compositions, manifested byfissuring and blistering upon tiring, have long beenproblems in theelectronics industry. These problems have been found to be related tothe particle shape and size of the gold powders used. The process ofthis invention has been discovered to control particle structure andthus provide a gold powder having a high level of brightness and goodfiring properties.

The conventional techniques mentioned above produce gold particleshaving irregular, uncontrolled size and shape, as noted, for example, inUS. Pat. No. 3,390,981, issued July 2 1968 to Lewis C. Hoffman. Untilnow, the commercially practicable preparation of gold powder particleshaving controlled, predictable size and shape and of the controlledmixture of flake and spherical particles has not been disclosed orpublished.

SUMMARY OF THE INVENTION This invention relates to a process forproducing precipitated gold powder having a controlled particle size andshape comprising reducing a gold salt solution with a reducing agentselected from the group consisting of I hydroquinone, and bromo, chloro,and lower alkyl substituted derivatives thereof, and mixtures thereof,(2) oxalic acid, the alkali metal salts of oxalic acid, and mixturesthereof, and (3) combinations of 1 and (2) in the presence of aprotective colloid, at a temperature within the range of 20-100C. Inaddition, metallizing compositions containing these gold powders arepart of the invention.

DETAILED DESCRIPTION OF THE INVENTION According to the process of thisinvention an acid gold chloride solution preferably at an elevatedtemperature is mixed and reacted with a particular reducing agent(s) inthe presence of a protective colloid to prevent the precipitated goldparticles from agglomerating. It is preferred to add the gold chloridesolution to the reducing agent(s) rather than vice versa. There may alsobe added an easily removable defoaming agent to control foaming causedby boiling and/or evolution of gaseous by-product from the decompositionof the reducing agent. The most suitable reaction temperature range isapproximately 50-l00C., and preferably approximately -97C.

A commercially practical source of gold for the precipitation reactionis a chloroauric acid solution, also known as a gold chloride or acidgold chloride solution. A suitable solution of this kind may be preparedaccording to well-known procedures by dissolving gold metal in aquaregia so as to produce a solution containing approximately 1-50 percentmetallic gold by weight.

Of the numerous reducing agents which will precipitate gold from suchsolution, oxalic acid has been found to produce the flake form of goldpowder having the desired size (1-50 microns across widest flat surface)when specified process ingredients and process conditions are utilized.As used herein, the term oxalic acid is meant to include water-solublesalts, such as alkali metal salts, of oxalic acid. On the other hand,hydroquinone has been found to produce spherical particles which are0.5-2 microns in diameter. As used herein, the term hydroquinone ismeant to include also substituted hydroquinones such as bromoandchloro-hydroquinone and lower alkyl substituents such as methylanddi'methyl-hydroquinone.

In order to produce the most desirable gold powders of this invention,it is necessary to use, as the reducing agent, hydroquinone togetherwith oxalic acid. The utilization of this combination of reducing agentsproduces either (I) a gold powder containing flakes and spheres or (2) agold powder containing spheres. The essential difference when using thiscombination of reducing agents lies primarily in the rate at which thegold chloride solution is added to the reducing agents. When the goldchloride solution is added rapidly, i.e., at least 500 cc./ minute, amixture of gold flakes and spheres is produced. When the gold chloridesolution is added slowly, i.e., less than 500 cc./minute, essentiallyonly gold spheres are produced. The gold chloride solution may be addedincrementally or continuously. The important requirement is that thetotal amount of gold chloride solution added per minute is within thelimits recited above.

Concurrently, the concentration of the gold in the gold chloridesolution is an important factor to be controlled in the process of thisinvention. As stated above, from 1-50 percent by weight of metallic goldshould be present in the gold chloride solution. For example, a workableconcentration contains approximately 400 grams of gold per liter of goldchloride solution, which represents about 30 weight percent gold.

The concentration of the reducing agent is not critical when eachreducing agent is used alone. However, it is preferred that the totalamount of reducing agent be in excess of that stoichiometricallyrequired to react with the gold, i.e., in order to bring the reactionessentially to completion. In using oxalic acid with hydroquinone, theratio of the oxalic acid to the hydroquinone is important. For bestresults the total amount of oxalic acid is approximately 5 times theamount of hydroquinone. Here also, the total quantity of reducing agentis not critical but should be in excess over the stoichiometricallyrequired quantity in order to bring the reduction reaction essentiallyto completion. Preferably, the hydroquinone will be about onefifth thestoichiometric amount required to precipitate all of the gold presentwhile the oxalic acid will be in excess.

Although agitation is not absolutely essential to the success of theprocess, some agitation is desirable, as in most chemical processes, tosecure good intermixing of the ingredients.

After reaction, the gold powder may be recovered from the slurry by anyof the common techniques for separating particulate solids from themother liquor. Appropriate techniques include filtration, decantation,and centrifugation. The recovered powder should be thoroughly washedwith water to remove by-products salts, residual protective colloid, anddefoaming agent (if used). The water wash should be followed by athorough wash with a water-miscible organic solvent, such as methanol,to remove all water and oxidation products of the reducing agents (e.g.,quinhydrone) and to facilitate drying. Drying may then be accomplishedby any convenient means. v

It has been found that maintenance of the particulate fonn ofprecipitate requires the use of a protective colloid to impedeagglomeration of the powder particulates and to protect and separate theflakes preventing agglomeration into larger sponge-like masses. Theconcentration of protective colloid useful in this invention isapproximately 1-15 grams/liter, and preferably 5-10 grams/liter. Of thenumerous protective colloids available, gum arabic was found to be'mosteffective. Other well known water-soluble gums and resins such as methylcellulose, sodium alginate, gum tragaeanth, gelatin, and the like werefound to be moderately effective and thus within the broad scope of theinvention.

The addition of even relatively small amounts of a protective colloid tothe acid gold chloride solution may tend to increase the solutionviscosity. Thus the evolution of gas from the oxidation of the oxalicacid may tend to cause frothing, which must be controlled. Anotherpossible cause of bubble and froth formation is localized overheatingand boiling of the reaction mixture. Any of numerous antifoam agentswould be effective in controlling the froth. However, residual amountsof antifoaming agents may tend to remain adsorbed on the gold particlesafter washing and lead to very poor firing properties of metallizingcompositions which are subsequently prepared from the gold powder. Thepreferred antifoam agents are therefore, those which do not remainadsorbed on the gold particles. Octyl alcohol is typical of this typeantifoam agent. Of course, other similarly removable defoaming agentswould also be useful; or mechanical, electrical, or ultrasonic meansknown to the art may be employed to break up the foam.

The invention is illustrated by the following examples. In the examplesand elsewhere in the specification all parts, percentages andproportions of material or components are by weight.

EXAMPLE 1 Into one liter of water were added 5.3 grams of gum arabic, 27grams of oxalic acid, and 2.7 grams of hydroquinone. The mixture wassubjected to mild agitation and heated to approximately 97C. Then 108grams of gold chloride solution (containing 27 grams of gold) wererapidly poured (within a few seconds) into the reducing agent. Due tobubbling produced by the gas evolved from the oxidation of oxalic acid,octyl alcohol was added in IO-drop increments as required to controlfoam formation. A precipitate began to form immediately and the reactionwas essentially complete in about 15 minutes; however, the mixture wasretained intact for approximately l hour to ensure completion of thereaction. The precipitated gold powder was filtered, washed with waterand methanol, and dried. Microscopic examination of the powder revealedspherical particles approximately 0.5-2 microns in diameter withrandomly interspersed hexagonal and triangular flakes approximately 1-15microns across their widest flat surface.

EXAMPLE 2 Into one liter of water were added 10 grams of gum arabic and200 grams of oxalic acid; the mixture was subjected to mild agitationand heated to about C. To this were added 10 drops of octyl alcohol tocontrol foaming. Then 120 grams of a solution of gold chloridecontaining 30 grams of gold were immediately poured into the mixture.There was an induction period of about 60 seconds between the additionof the gold solution and the first appearance of a gold precipitate;however, the reaction was essentially complete after about 15 minutes asevidenced by termination of bubble evolution. The hot gold slurry wasfiltered through a sintered glass funnel and the precipitate wasthoroughly washed on the funnel with water to remove chlorides, gumarabic and the unreacted oxalic acid. The precipitate was then washedthoroughly with methanol to remove water and was dried at roomtemperature. A microscopic examination of the powder showed that it wascomprised of hexagonal and triangular plates 5-15 microns across thewidest flat surface. There were also many small, similarly shapedparticles in the range of l-5 microns, but the greater portion was inthe larger particle range.

EXAMPLE 3 Into one liter of water were added 10 grams of gum arabic andgrams of hydroquinone. The mixture was subjected to mild agitation andheated to approximately 85C. To this was added 200 grams of a goldchloride solution containing 50 grams of gold. A precipitate began toform immediately and the reaction was essentially complete in about 15minutes. The gold slurry was filtered hot, washed thoroughly with water,and then with methanol, and dried at room temperature. Microscopicexamination of the powder showed it to be comprised of very uniformspherical particles about 0.5-2 microns in diameter, the greater portionbeing about 1 micron in diameter.

EXAMPLE 4 cohol were added. After the reaction subsided, the

precipitated gold was allowed to settle. The batch was then decantedthrough a sintered glass filter to catch the fines, and washedsuccessively with hot water and methanol. The gold powder was driedfirst at room temperature and then at 90C. to remove methanol and anyresidual water.

A microscopic examination of the resulting powder showed a preponderanceof 1 micron spheres of gold but with a large quantity, visuallyestimated at about 20 percent of 5-15 micron flakes.

EXAMPLE 5 The process of Example 4 was repeated in every detail exceptthat the gold chloride solution was added in 250 ml. increments every 30seconds until the 1,500 ml. had been added, requiring 3 minutes forcomplete addition. Microscopic examination of this powder showed that atleast 98 percent of the gold was in the form of 1 micron spheres andonly occasional flakes were seen.

The gold powder of this invention can be used to prepare variousmetallizing compositions such as are employed in printed circuits,resistors, and other electronic applications. Such metallizingcomposition may be readily formulated according to well-known proceduresas are described in U.S. Pat. Nos. 3,413,240, 3,385,799 and 3,347,799.Of course, formulation of metallizing composition will depend uponend-use requirements. Such relevant factors as liquid vehiclecomposition, inorganic binder (e.g., glass frit), gold content, thekinds and amounts of other metals (e.g., platinum, palladium, silver,ruthenium), and firing conditions will be determinable bystraightforward experimentation. A useful metallizing composition isdescribed in Example 6.

EXAMPLE 6 A metallizing composition was prepared from the gold powder ofExample 1. The composition contained 84 percent gold powder, 2 percentof a finely divided lead-borate glass frit composed of approximately 83percent lead oxide and 17 percent boron trioxide, and 14 percent of avehicle consisting of percent hydrogenated rosin, 10 percent ethylhydroxyethyl cellulose, 40 percent beta-terpineol, 20 percent keroseneand 20 percent aliphatic hydrocarbon fraction boiling higher thankerosene. This composition which was printed and fired using ordinarytechniques, produced tight, bright printed lines having good electricalproperties.

We claim:

1. A process ,for producing precipitated gold powder having a controlledparticle size and shape comprising reducing a gold salt solution with areducing agent selected from the group consisting of (l) hydroquinone,and bromo, chloro, and lower alkyl substituted derivatives thereof, andmixtures thereof, (2) oxalic acid, the alkali metal saltsof oxalic acid,and mixtures thereof, and (3) combinations of (l) and (2) in thepresence of a protective colloid, at a temperature within the range of20-l00C. g

2. A process in accordance with claim 1 wherein the protective colloidis gum arabic.

3. A process in accordance with claim 1 wherein the precipitated goldpowder consists essentially of gold flakes, said flakes beingapproximately l-50 microns across their widest surface, wherein the goldis precipitated from an acidic gold chloride solution with a reducingagent selected from the group consisting of oxalic acid, the alkalimetal salts of oxalic acid, and mixtures thereof, in the presence of aprotective colloid, at a temperature within the range of 50-100C.

4. A process in accordance with claim 3 wherein the protective colloidis gum arabic.

5. A process in accordance with claim 3 wherein the gold flakes areapproximately l-l5 microns across their widest surface and the reducingagent is oxalic acid.

6. A process in accordance with claim 1 wherein the gold powder containsrelatively spherical particles approximately 0.5-2 microns in diameter,wherein the gold is precipitated from an acid gold chloride solutionwith reducing agent selected from the group consisting of hyd'roquinone,and bromo, chloro, and lower alkyl substituted derivatives thereof, andmixtures thereof, in the presence of a protective colloid, at atemperature within the range of 50-l 00C.

7. A process in accordance with claim 6 wherein the protective colloidis gum arabic.

8. A process in accordance with claim 7 wherein the temperature isapproximately -l00C.

9. A process in accordance with claim 1 wherein the gold powder consistsessentially of a mixture of gold flakes approximately l-50 micronsacross their widest surface and relatively spherical gold particlesapproximately 0.5-2 microns in diameter, wherein the gold isprecipitated from an acid chloride solution by reaction with reducingagents comprising (a) at least one member selected from the groupconsisting of oxalic acid and the alkali metal salts of oxalic acid, and(b) at least one member selected from the group consisting ofhydroquinone, bromo, chloro, and lower alkyl substituted derivativesthereof, in the presence of a protective colloid, at a temperaturewithin the range of 50-100bLC.

10. A process in accordance with claim 9 wherein the protective colloidis gum arabic.

11. A process in accordance with Claim 10 wherein the temperature isapproximately 85-l00C.

12. A process in accordance with Claim 10 wherein the gold flakes areapproximately l-15 microns across their widest surface and the reducingagent is a mixture of hydroquinone and oxalic acid.

13. A process in accordance with claim 12 wherein the ratio of oxalicacid to hydroquinone is approximately 10 to l on a weight basis.

14. A process in accordance with claim 9 wherein foaming and frothingare controlled by the presence of a defoaming agent.

15. A process in accordance with claim 14 wherein the defoaming agent isoctyl alcohol.

16. A process in accordance with claim 1 wherein the gold powderconsists essentially of relatively spherical gold particlesapproximately 0.52 microns in diameter, wherein the gold is precipitatedfrom an acid gold chloride solution by reaction with reducing agentscomprising (a) at least one member from the group consisting of oxalicacid and the alkali salts of oxalic acid, and (b) at least one memberselected from the group consisting of hydroquinone, bromo, chloro, and

lower alkyl substituted derivatives thereof, in the presence of aprotective colloid, at a temperature within the range of 50-l00C., saidreaction being performed by slowly adding the gold chloride solution tothe reducing agent.

17. A process in accordance with claim 16 wherein the gold chloridesolution is added to the reducing agent at the rate of 1-500 cc. perminute.

18. A process in accordance with claim 17 wherein the gold chloridesolution is added in increments over a l-hour period.

19. A process in accordance with claim 17 wherein the gold chloridesolution is added continuously.

2. A process in accordance with claim 1 wherein the protective colloidis gum arabic.
 3. A process in accordance with claim 1 wherein theprecipitated gold powder consists essentially of gold flakes, saidflakes being approximately 1-50 microns across their widest surface,wherein the gold is precipitated from an acidic gold chloride solutionwith a reducing agent selected from the group consisting of oxalic acid,the alkali metal salts of oxalic acid, and mixtures thereof, in thepresence of a protective colloid, at a temperature within the range of50*-100*C.
 4. A process in accordance with claim 3 wherein theprotective colloid is gum arabic.
 5. A process in accordance with claim3 wherein the gold flakes are approximately 1-15 microns across theirwidest surface and the reducing agent is oxalic acid.
 6. A process inaccordance with claim 1 wherein the gold powder contains relativelyspherical particles approximately 0.5-2 microns in diameter, wherein thegold is precipitated from an acid gold chloride solution with reducingagent selected from the group consisting of hydroquinone, and bromo,chloro, and lower alkyl substituted derivatives thereof, and mixturesthereof, in the presence of a protective colloid, at a temperaturewithin the range of 50*-100*C.
 7. A process in accordance with claim 6wherein the protective colloid is gum arabic.
 8. A process in accordancewith claim 7 wherein the temperature is approximately 85*-100*C.
 9. Aprocess in accordance with claim 1 wherein the gold powder consistsessentially of a mixture of gold flakes approximately 1-50 micronsacross their widest surface and relatively spherical gold particlesapproximately 0.5-2 microns in diameter, wherein the gold isprecipitated from an acid chloride solution by reaction with reducingagents comprising (a) at least one membEr selected from the groupconsisting of oxalic acid and the alkali metal salts of oxalic acid, and(b) at least one member selected from the group consisting ofhydroquinone, bromo, chloro, and lower alkyl substituted derivativesthereof, in the presence of a protective colloid, at a temperaturewithin the range of 50*-100*C.
 10. A process in accordance with claim 9wherein the protective colloid is gum arabic.
 11. A process inaccordance with Claim 10 wherein the temperature is approximately85*-100*C.
 12. A process in accordance with Claim 10 wherein the goldflakes are approximately 1-15 microns across their widest surface andthe reducing agent is a mixture of hydroquinone and oxalic acid.
 13. Aprocess in accordance with claim 12 wherein the ratio of oxalic acid tohydroquinone is approximately 10 to 1 on a weight basis.
 14. A processin accordance with claim 9 wherein foaming and frothing are controlledby the presence of a defoaming agent.
 15. A process in accordance withclaim 14 wherein the defoaming agent is octyl alcohol.
 16. A process inaccordance with claim 1 wherein the gold powder consists essentially ofrelatively spherical gold particles approximately 0.5-2 microns indiameter, wherein the gold is precipitated from an acid gold chloridesolution by reaction with reducing agents comprising (a) at least onemember from the group consisting of oxalic acid and the alkali salts ofoxalic acid, and (b) at least one member selected from the groupconsisting of hydroquinone, bromo, chloro, and lower alkyl substitutedderivatives thereof, in the presence of a protective colloid, at atemperature within the range of 50*-100*C., said reaction beingperformed by slowly adding the gold chloride solution to the reducingagent.
 17. A process in accordance with claim 16 wherein the goldchloride solution is added to the reducing agent at the rate of 1-500cc. per minute.
 18. A process in accordance with claim 17 wherein thegold chloride solution is added in increments over a 1-hour period. 19.A process in accordance with claim 17 wherein the gold chloride solutionis added continuously.